(1) Field of the Invention
The present invention relates to an optical pickup device of an optical disk unit.
(2) Description of the Related Art
In order to increase the access speed of an optical disk unit, it is necessary to reduce the weight of a movable part of the optical pickup device, and it is preferred that only a part of a necessary minimum function be movable and other parts be fixed.
As the means for reducing the weight of the movable part in the optical pickup device, there has been known a method in which a rotary moving mirror is arranged as the tracking means on a fixed optical system (see "Collection of Lectures at Spring Meeting, 1985 of Japanese Association of Precision Machines, No. 146").
More specifically, this optical pickup device comprises a moving optical system having a focussing object lens confronting a disk-shaped information-recording medium and moving (making a stroke) linearly in the radial direction of the information-recording medium, and a fixed optical system secured to the device body and comprising a light source, a beam splitter, a collimator and an optical detector arranged on an optical path divided by the beam splitter, wherein a rotary movable mirror having a rotation angle controlled for tracking is arranged as a part of the fixed optical system between the moving optical system and the fixed optical system.
In this optical pickup device of this type, the tracking quantity is determined by the deflection angle of the beam incident on the object lens. When this deflection angle is kept constant, the track offset quantity is in proportion to the deflection quantity of the beam on the focal plane (the plane vertical to the optical axis at the focus of the optical system) of the object lens on the beam-incident side. The deflection of the beam caused by rotation of a rotary reflecting mirror increases according to the distance from the rotary reflecting mirror and especially increases at the stroke end of the moving optical system.
Accordingly, in the method in which an ordinary movable mirror having the center of rotation in the vicinity of the mirror surface is merely separated to the side of the fixed optical system, a large track offset is produced when the moving optical system separates from the rotary movable mirror.
Furthermore, a method in which another moving means such as a parallel plane plate rotation actuator is disposed to correct this offset is defective in that means for controlling this actuator becomes necessary and the structure becomes complicated.